In direct injection spark ignition engines, the engine operates at or near wide-open throttle during stratified air/fuel operation in which the combustion chambers contain stratified layers of different air/fuel mixtures. The strata closest to the spark plug contains a stoichiometric mixture or a mixture slightly rich of stoichiometry, and subsequent strata contain progressively leaner mixtures. The engine may also operate in a homogeneous mode of operation with a homogeneous mixture of air and fuel generated in the combustion chamber by early injection of fuel into the combustion chamber during its intake stroke. Homogeneous operation may be either lean of stoichiometry, at stoichiometry, or rich of stoichiometry.
Direct injection engines are also coupled to conventional three-way catalytic converters to reduce CO, HC, and NOx. When operating at air/fuel mixtures lean of stoichiometry, a NOx trap or catalyst is typically coupled downstream of the three-way catalytic converter to further reduce NOx.
Direct injection engines are also coupled to fuel vapor recovery systems to allow purging of fuel vapors. Conventional systems allow fuel vapor purging in the stratified mode only when the catalyst temperature is high enough to convert the unburned hydrocarbons. In other words, since the fuel vapor is inducted with the fresh charge, it forms a homogenous air/fuel mixture. Then, when the fuel is directly injected during the compression stroke to form a stratified mixture, only the stratified fuel burns since the homogenous fuel vapor mixture is too lean. Therefore, unburned fuel vapor may exit the engine cylinder. In some circumstances, to remove these vapors, the catalytic converter may have to be above a certain temperature, known to those skilled in the art as the light-off temperature. Therefore, fuel vapor purging during the stratified mode is restricted until the catalyst has reached this light-off temperature. Such a method is described in U.S. Pat. No. 5,245,975.
The inventor herein has recognized a disadvantage with the above approach. When operating in the stratified mode, less heat is generated to increase or maintain catalyst or NOx trap temperature. With some catalyst formulations and catalyst configurations, the catalyst may not maintain this light-off temperature during certain conditions and purging is only possible in the homogeneous mode, thereby reducing fuel economy and allowable purge time. In other words, when operating in the stratified mode, catalyst or NOx trap temperature may fall below the light-off temperature, preventing purge and thereby limiting stratified operation.